Prime-time winter Wednesdays are about to be filled with Roman legions spreading their eagles and Roman women being told to spread their legs. It will be a long haul because, unlike Caesar's Gaul, the HBO-BBC series Rome is divided into 12 parts. It deserves to start with a big audience because the faces are excellently cast, the sets and clothing are outstanding and the lighting and look are stunning. It is splendidly ambitious and focuses on major events in world history. Five years and $100m have been well spent.

Contrary to the classicists' fears, it is not all oiks and orgies. Sex scenes and sword-violence push the BBC's rules to the limit in episode one but the senatorial class of British actors projects a fine British accent throughout. We knew that the young Octavian, the future emperor, Augustus, was the grandson of a municipal banker, but until tonight you may never have guessed that he went to school at Harrow.

The storyline covers the fall of Rome's free republic in the late 50s and 40s BC to the ambitions of Julius Caesar, the prelude to the transforming era of Roman emperors. Ciaran Hinds has just the right patrician blend of charm and hard superiority to be pretty convincing as Julius Caesar; his rival, the great Pompey, is played with plausible realism by Kenneth Cranham and dominates the senators in Rome. The women are all on top of their form and - spectacularly often - on top of their men. Polly Walker is hard at it as Atia, Caesar's niece, but my bet is on Lindsay Duncan to be the star of the series as Brutus's mother, Servilia. Her son Brutus strikes a false note with the sort of language, drawl and manner that belongs at a public school, not with Rome's "noble Brutus'. In general, though, the actors have done very well with the script and scenes which have been handed to them.

Actually, the scriptwriter, Bruno Heller, has hit a plausible tone and sustained it with just enough clangers to keep scholars busy. (Fancy calling a republican woman the "crucifix of Venus".) His brief was to integrate powerful events round the rich and powerful with a drama of everyday participants, a mixture of Newsnight with Desperate Housewives in the seedy Suburra district of Rome. The backstreets have been memorably set and worked over by the camera. But it is the combination of these two levels which makes the entire venture starts to wobble.

Aptly, we begin in 52BC when Julius Caesar is victorious in Gaul, his command there is beginning to run out and the question of his ambitions is becoming ever more urgent in Rome. But we are starved of the historical context which would explain and deepen this crisis. All we get is Pompey v Caesar, fired up by some unhistorical subterfuges from Pompey and the ominous sound of a military officer blowing a whistle, surely an anachronism, when Caesar's men first engage.

We wait far too long for even an oblique reference to the tremendous issues which were the setting to the two dynasts' conflict. We are never really told that Caesar had outraged traditionalist senators by his previous consulship in 59 BC. Most of them were terrified that he would return from Gaul and become consul a second time and be even more appallingly populist. Freedom, tradition and an execration of one-man rule were in the senators' bloodstream.

But the scenes in the senate house are telescoped and turned into a disorderly travesty. Significantly, the one miscasting is the arch-traditionalist, Cato, who looks as if he has escaped into the community from a care home beyond the Tiber.

As a result, the grand picture is boringly trivialised. People watch historical drama because they really do want to pick up some of the history apart from the colour of the plastered houses and the style of the soldier's boots.

Nobody wants to be swamped with constitutional problems, but a sense of context gives a sense of the choices and dilemmas which people faced at the time. The drama has been almost totally depoliticised, which is patronising to the audience. Caesar was a master of spin and half truths, but when we hear his proclamations by letter to Rome, we have no idea of his political cunning. In reality, he represented himself as the defender of the people's "liberty" and the enemy of a "faction" (who were actually the majority of the senate).

By episode two, Caesar will be crossing the Rubicon, probably the most famous thing he ever did. He made it into such a dramatic moment, the start of a civil war which would eventually overthrow a free political constitution that had survived for 400 years. Here, we simply see him splashing through the river, watched by an ordinary youth who is fishing. Actually, he slipped out from a dinner party after watching a gladiatorial show in the evening. He said, notoriously, the "die is cast".

He is said to have reflected on the immense evil for the human race which his actions would initiate. It is wretched to see this great moment betrayed. Caesar's climax is a visual flop because it is historically hollow. Down in the ranks and the rancid back streets of Rome, the everyday plots are going to have to pick up momentum. So far, we have had a dodgy paternity case, a gruesome stab through the throat and more flagrante delicto in a brothel.

The sub-plot is going to turn on two matey soldiers, one of whom is ominously named Pullo. I suspect that their everyday soap drama is intended to show these stirring times at a level with which we can all inter-relate, but its plotlines are too detached from the main drama of the civil war. It is here that the series drifts furthest apart from the triumphant I Claudius of the 1970s.

It is not just that I, Claudius had the inimitable books of Robert Graves behind it. It showed the unforgettable carrying-on in the imperial family when events in the household were directly inter-linked with public affairs and the bigger imperial plot. There was no need for ordinary men and a Pullo of a sub-story.

My fear is that Rome has detached the everyday stuff from the big issues which affected even the way in which such everyday events would develop. Episode two is already much flatter than tonight's vivid sequences of sex and slaughter. I am not blaming the actors, but I do blame a conception which has run out at the last fence and thrown too much fascinating history off the set.

It is a great pity because the production teams and designers deserve a roar of applause. They give a tremendous impression of Roman houses and the murky danger in city life. The protagonists' attitude to their slaves in the first 20 minutes is as instinctive and arresting as anything in that touchstone of Roman film-history, Spartacus.

Among so much that is good, the problem remains the dilution of one of the world's most inspiring and controversial stories. I fear the worst and I do wonder about the details. Do we really know that a noble Roman matron would climb off her male lover, in full view of her slaves? I refuse to believe that she would then go off and be showered with the blood of a sacrificed bull, a rite which we can only document in Italy some 200 years later.

History is far less dull than a mundane image of the "everyday". By throwing it out, the makers of the series have ended up with an exotic falsehood.

· Rome is on BBC2 tonight at 9pm. The Classical World, An Epic History from Homer to Hadrian, by Robin Lane Fox, has just been published by Allen Lane, priced £25